This disclosure relates to high accuracy RC oscillators.
Oscillators are the primary clock sources, for example, in microcontrollers and other integrated circuits. The number of oscillators can be increased to obtain the required frequencies, for example, by means of phase locked loops. The clock accuracy, however, is limited by the oscillator source.
Internal RC oscillators are attractive for microcontroller and other applications because of their relatively low cost. They can be highly integrated with other on-chip components and generally require no external components. However, the accuracy of RC oscillators usually does not exceed a few percent, which limits their use to non-critical clocks.
Many applications with accurate clock requirements incorporate a highly accurate crystal oscillator. To incorporate the crystal oscillator, such applications typically require multiple input/output pins and external components. Although such requirements are undesirable in microcontroller and other applications due to space and cost constraints, suitable alternatives have traditionally been unavailable.
Some applications require clocks with a tolerance better than can be provided by an RC oscillator (i.e., a few percent), but not as accurate as a crystal oscillator (i.e., tens of parts per million (ppm)). For example, a universal serial bus (USB) at full speed may require a clock accuracy of 2500 ppm. A crystal oscillator often is used to meet these clock requirements.
In low power modes, one oscillator can be operated continually to wake up the system at periodic intervals. An ultra-low power (ULP) RC oscillator can be used. However, as such an oscillator typically is highly inaccurate, it only allows the system to wake up at approximate intervals. In view of the relatively poor accuracy of a ULP RC oscillator, it typically is not useful as a reference for frequency multipliers. For applications in which a ULP RC oscillator is not sufficiently accurate, a 32 kHz crystal oscillator may be used, but further increases the cost of the system.